If \(P(x,y) = h(a(x)+b(y))\)  or \(P(x,y) = h(a(x)b(y))\) , where \(h,a,b\)  are univariate real polynomials, then we call \(P\)  a (polynomial) special form. If \(P\)  is not a (polynomial) special form then we call it an expanding polynomial. Theorem REF  has since been generalized in several directions. See [1]}, [2]}, [3]}, [4]}, [5]}, [6]}, [7]}, and the references therein.
